Dimensionally precise objects/articles find numerous applications in electronics, optics, automotive, aerospace, and other high-technology industries. Examples include optically transparent objects/articles such as various precision lenses (spherical and aspherical), ophthalmic lenses (single vision, bifocal, trifocal, and progressive), contact lenses, optical data storage disk substrates, and projection optics/lens arrays. Non-transparent but dimensionally exact parts abound, such as couplers, housings, gears, and various packaging assemblies. The most straightforward fabrication method for dimensionally precise parts is the machining, grinding, and polishing of sheet stock and, in fact, this approach is still used today for some types of ophthalmic lenses. Unfortunately, this approach is limited to simple geometries and is costly due to the relatively large amounts of skilled labor required to produce a single part. More commonly, the plastics industry relies on well-known processes such as injection molding, compression molding, transfer molding, reactive injection molding (RIM), and casting for the fabrication of geometrically complex parts.
Injection molding, compression molding and transfer molding require the use of thermoplastic polymers. Material choices are limited to uncrosslinked polymers that can be melted by heat and injected at high pressures. Example polymers include polymethylmethacrylate, polystyrene, ABS (acrylonitrile-butadiene-styrene) and polycarbonate. These molding processes entail high temperature and pressure; therefore, expensive molding equipment and molds are necessary. Large parts with thick cross-sections are difficult to mold, since the heat transfer rate is slow. Long cycle times make the processes uneconomical. Additionally, finite coefficients of thermal expansion can lead to part warpage upon cooling. Thus, these processes are seldom practical for large-scale manufacturing of truly dimensionally demanding parts.
Reaction injection molding requires the use of at least two highly reactive components (A+B). Urethane is one such example, where the reactive components are monomeric isocyanates and alcohols. The components are quickly and thoroughly mixed just before injection into the mold cavity. The material is then allowed to quickly set in the cavity (cured). This methodology relies on materials that are highly reactive and generally toxic. Mechanical means for thorough mixing is part of the integral process, making the production equipment costly. The fabricated parts are also not quite dimensionally exact due to shrinkage effects associated with the polymerization process. Material selection is limited by the required reaction chemistry, as well. Thus, reactive injection molding (RIM) is limited by the need for highly reactive functional groups, the intensity of mixing prior to mold fill, and complex and expensive machinery to carry out the process.
Fabrication of precision parts has been attempted by processes generally known as casting. Casting is typically a less expensive alternative to the above processes. It is also a more flexible process, in that a great number of precursor mixtures (e.g., monomers, crosslinkers, oligomers, etc.) can be formulated to achieve different final parts and performance properties. The final parts can be thermosets, formed by a polymer network that is crosslinked to prevent melt flow. Since the precursor solution has a relatively low viscosity to facilitate mold fill, the process is a low-pressure operation, reducing the necessary equipment cost. The casting process, unfortunately, is often compromised by the high shrinkage rate of the formulated precursor mixtures, yielding inexact parts with warped shapes. The high shrinkage rate is a natural consequence of using precursors that have low to moderate viscosities. If we take an ophthalmic lens as an example, the mold defining the lens-shaped cavity generally consists of a front and a back half, and an intervening gasket. The front mold half is concave, whereas the back mold half is convex. Detailed design features distinguish the utility of the resulting lens. Hence, simple vision, bifocal, trifocal, progressive, spherical, aspherical, and toroidal lenses can all be made, in principle, but if and only if the in-situ curing process can be performed with near net-shape fidelity. This is obviously a difficult task, at best, if the material used for casting exhibits a high degree of shrinkage. Casting thus requires a mold-filling step, an activation step to trigger and sustain polymerization, and a mold-opening/ensuing cleaning step to finish the part and to recycle/re-shelf the mold halves. To date, all known casting processes begin with a polymerizable fluid that can be easily fed into the mold cavity, i.e., at moderate pressures. Care is necessary to minimize bubble creation. A carefully designed gasket is required, applied to seal off the cavity formed by the mold halves. Then a controlled curing step is imposed to convert the liquid feed into a finished, solid object.
Most curable formulations contain carbon-carbon double bonds. Such unsaturated sites are exemplified by functional groups like acrylates, methacrylates, vinyl ethers, and vinyls. Free radical or ionic polymerization mechanisms can be induced by the appropriate initiators, triggered by either UV or heat, i.e., photo- or thermally-induced polymerization. Since the reaction mixtures must fill the cavities in as short a time as possible to allow reasonable process economies, small-molecule or oligomeric mixtures are usually employed to keep viscosities low. These systems have a significant degree of shrinkage upon cure, as high as 15% for some oligomeric mixtures, and even greater than 20% for some small molecule formulations.
Additionally, the polymerization of unsaturated species is unfortunately an exothermic reaction. When such a reactive system is cured a great deal of heat is generated. The result is a spurious temperature excursion of the cast part during cure which often leads to thermal degradation of the material, discoloration, and part warpage upon cool down and removal from the mold. This problem may be reduced by improving heat transfer. Unfortunately, heat transfer can be improved only so much, due in part to the poor thermal conductivity of most polymeric systems.
Overheating during cure can also be reduced by lowering the concentration of initiator species in the starting formulation, except that decreasing the initiator concentration prolongs the curing process and can lead to incomplete curing reactions and only partially polymerized final objects.
The heat generation and shrinkage accompanying polymerization must be accommodated by specially engineered curing processes, such as zone-curing techniques, in order to produce exact parts that replicate the contours of the cavity, and to slow the curing reaction so as to reduce spurious temperature rises. The need to use a gasket to prevent leakage (material escape) and minimize introduction of air bubbles dictates limited flexibility of mechanical design. It is also difficult to have the front and back mold halves positioned in such a way so as to intentionally create a non-aligned axial offset (known as de-centration). In addition, it is difficult to have the two axes rotated to create an intentional tilt (thus introducing a prismatic effect to the ensuing lens).
Finally, since most if not all of the reactive mixture exists initially in an unpolymerized state, the process must accomplish the curing of all such as-yet unreacted material precursors so that no small-molecule, volatile species remain in the finished part. This has the effect of protracting the process duration, especially if the initiator composition is kept low so as to minimize rate of heat generation. In free radical polymerization, this problem is further exacerbated by the reaction inhibition which occurs as a result of the presence of oxygen (either dissolved in the polymerizing liquid, or present in the vapor space surrounding the mold). Nitrogen purging of both the polymerizing liquid and of the mold cavity must be employed to keep oxygen levels low so that polymerization may occur in a timely fashion. Often nitrogen purging is not able to remove all oxygen, and parts remain only partially cured, especially near the part surfaces, leading to sticky or tacky skins. Manufacturers have gone to great lengths in order to prevent oxygen from slowing the cure reaction in the near-surface region of cast parts, often employing initiators that react with oxygen diradicals, high levels of initiators (which increases the likelihood of high-temperature excursions and yellowing), or oxygen impermeable films at the surfaces of the cured parts. Insertion of such films entails opening the mold after partially curing the object, which further has the effect of complicating the process and protracting the process duration.